The present invention relates generally to stators for eccentric spiral pumps.
More particularly, it relates to a stator for an eccentric spiral pump, which has a thread-like profile on its inner surface and includes a metallic casing and an insert of a rubber-elastic material in the casing.
Eccentric spiral pumps are known in many forms and serve to deliver, for example, solid/liquid mixtures such as for example mortar, mud or the like. They include a stator which is provided with a thread-like profile on its inner surface and composed of a rubber-elastic material, and a rotor which is composed of steel and has a thread-like profile on its outer surface and rotates eccentrically. Delivery chambers for the medium to be delivered are formed between the threaded profiles of the stator and the rotor. The chambers are sealed off from one another and moved in the delivery direction during rotation of the rotor so as to change continuously their shape and position, but not their volume during the movement. The pressures or delivery heads achievable on the outlet side depend substantially on the quality of the seal between the delivery chambers and hence, inter alia on the rigidity or dimensional stability of the rubber profile of the stator. The latter is usually adjusted by way of certain initial stress that is the difference in diameter between the rotor and the stator. A comparatively softer rubber material in the frame of the stator profile is considered to be more wear-resistant than a relatively harder one. However, the former requires substantially higher initial stress to achieve the same outlet pressure, such that, since rubber should be considered as incompressible material the result would be comparatively marked deformations of the stator profile.
There are limits to increasing the initial stress with the aim of increasing the outlet pressure both with regard to the necessary torque and with regard to the wear which occurs. This wear has an effect inter alia, on the steel rotor. The steel rotor commencing on the pressure side and continuing in the direction of the suction side, becomes worn and its dimensions finally take on the shape which tapers in general conically toward the pressure side. This gradual material wear results in a continuously decreasing initial stress and sealing effect between the delivery chambers, and hence a reduction in the achievable delivery head. Increasing performance by increasing speed would not be useful either, in view of the wear to be expected and the energy requirements.
Stators for eccentric spiral pumps conventionally have a cylindrical metal casing and an insert introduced in the casing by an injection process and made of rubber material. The insert has on the inside the shape of for example a double-threaded profile intended for cooperating with the rotor. It is known that in rubber materials the amount of shrinkage to be expected is dependent upon the respective wall thickness of the profile. With regard to achieving low production tolerances special additional measures therefore are required in order to meet the tolerances despite the locally varying wall thickness which are attributable to the threaded profile.
German reference DE-AS 1,553,199 discloses a readjustable stator for an eccentric spiral pump. In the stator the insert composed of an elastic material is located in a metallic casing, the diameter of which is reducable so that any wear occuring can be compensated within a certain scope by way of a reduction in diameter. Problems arise however in the precise adjustment of the respective initial stress required which at least when used on building sites where this is at all possible, means a considerable outlay.
In order to reduce the friction power losses caused by the initial stress between the stator and the rotor, it was proposed in the German document DE-PS 3,304,751 to continuously reduce the effective dimensions of the stator starting from its suction side up to its pressure side. In conjunction with the rotor which, as seen in the axial direction has the same radial dimensions, maximum initial stress is available here on the pressure side. In this way a reduction in torque can be achieved.